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Kitchen chemistry: fats and oils

Our previous kitchen chemistry post discussed esters. Fats and oils (triglycerides) are an important special case of esters. The alcohol in triglycerides is glycerol, a “triple alcohol” with three OH groups:

The glycerol combines with “fatty acids” (like the one on the right) which resemble acetic acid (left), but with a much longer hydrocarbon chain hanging off the COOH group:

The resulting triglyceride esters have three COO groups:

Fatty acids have important dietary implications, and they can be classified in dietary terms, but the most common classifications are chemical. The three main chemical classifications all refer to the presence of carbon-carbon double bonds:

One important classification is in terms of the number of carbon-carbon double bonds:

Saturated fatty acids have no carbon-carbon double bonds (they are “saturated” in the sense of containing as much hydrogen as possible). Fats made from saturated fatty acids (“saturated fats”) tend to be solid at room temperature, because the straight-line molecules stick to each other. Saturated fats are usually of animal origin (although coconut oil and palm oil are also mostly saturated).

The position of carbon-carbon double bonds is also significant. A common classification counts the position of the first double bond, starting from the “omega” end of the molecule (the end furthest from the oxygen atoms). For example, there are omega-3, omega-6, and omega-9 fatty acids:

Finally, the orientation of double bonds is very important. In cis fatty acids, there are two hydrogen atoms on the same side of the double bond, giving a molecule with a “kink.” In trans fatty acids, the two hydrogen atoms are on the opposite sides of the double bond, giving a straight-line molecule (trans fats are usually synthetic, resulting from the partial hydrogenation of vegetable oils). Since the straight-line molecules tend to stick together, “trans fats” (made from trans fatty acids) tend to be solid at room temperature, while “cis fats” (made from cis fatty acids) tend to be liquid – that is, oils (such as olive oil) rather than fats:

Fatty acids can also be classified in dietary terms. The body needs fatty acids, but can manufacture most of them itself. Essential fatty acids are fatty acids that must be included in the diet. Both ALA and linoleic acid (found in vegetable oils) are essential. Adult men need about 13 grams of linoleic acid and 1.3 grams of ALA per day. Some omega-3 fatty acids from oily fish should also be included in the diet.

In contrast, trans fats are particularly unhealthy, and should be eliminated from the diet completely. This can be difficult in the USA, since pre-packaged foods there often contain trans fats (because of their long shelf life). Monounsaturated and polyunsaturated oils are also preferable to solid saturated fats.